1. Field of the Invention
From German Patent Disclosure DE 196 50 865 A1, a magnet valve for controlling the fuel pressure in a control chamber of an injection valve, for instance for a common rail injection system, is known. By way of the fuel pressure in the control chamber, a reciprocating motion of a valve piston is controlled, with which an injection opening of the injection valve is opened or closed. The magnet valve includes an electromagnet, a movable armature, and a valve member, which is moved with the armature and is urged in the closing direction by a valve closing spring and cooperates with the valve seat of the magnet valve and thus controls the outflow of fuel from a control chamber.
2. Description of the Prior Art
In a currently used, leak-free fuel injector that is actuated by means of a magnet valve, the coupling between a valve piston and an injection valve member embodied in needle-like form is effected via a hydraulic coupler. The hydraulic coupler includes a coupler sleeve with an inner bore, in which the valve piston is guided. The diameter of the coupler sleeve is greater than the outside diameter of the injection valve member embodied in needle-like form. The coupler sleeve, on its lower end, rests with a sealing edge, embodied on its face end, on a nozzle body and thus encloses a coupler volume. In the state of repose, the coupler sleeve is positioned against an end face of the nozzle needle with a slight force, exerted by way of a spiral spring. The coupler sleeve or coupler is surrounded by fuel that is at system pressure. System pressure is understood to mean the fuel pressure level that is generated in a fuel injection system, for instance via a high-pressure pump, inside a high-pressure reservoir body (common rail).
If the fuel injector is triggered, then first the valve piston moves upward. This upward motion creates an underpressure in the coupler volume, compared to the system pressure level outside. Because of the underpressure, the injection valve member embodied in needle-like form follows the valve piston and as a consequence contacts the face end of the valve piston that is diametrically opposite the injection valve member which is preferably embodied in needle-like form. As the valve piston stroke becomes longer, the pressure in the coupler volume drops, since because of the pressure difference between the inner bore of the coupler sleeve and the outside diameter of the injection valve member embodied in needle-like form, the available fuel volume in the coupler increases. After the end of the triggering, the valve piston and the injection valve member embodied in needle-like form move downward again, in the closing direction. When the injection valve member embodied in needle-like form approaches its seat, the hydraulic force exerted from below on the injection valve member embodied in needle-like form drops, and the needle-like injection valve member leads ahead of the valve piston in the closing direction. Because of the fact that during the reciprocating motion fuel has flowed on into the coupler volume via the guidance play, the pressure in the coupler already reaches the system pressure before the valve piston is again in contact with the face end of the injection valve member embodied in needle-like form. Consequently, an overpressure comes about inside the coupler, causing the coupler sleeve to be lifted, counter to the slight prestressing force, from the face end of the nozzle body against which it is positioned, so that the trailing flow volume escapes again.
To avoid dynamic pressure differences between the coupler volume and the surrounding fuel, the coupler sleeve is guided on the valve piston with a comparatively great guidance play, on the order of magnitude of several micrometers, such as 8 μm, and over a length of several millimeters, such as 5 mm. The inside diameter of the coupler sleeve is approximately 3.8 mm, and the outside diameter of the injection valve member embodied in needle-like form is 3.5 mm. This layout causes the coupler pressure to trail the system pressure in the state of repose by the order of magnitude of 100 μs. By way of this play, during the reciprocating motion of the valve piston—as mentioned above—a quantity of fuel follows. Since after each injection the coupler sleeve lifts from its contact face on the nozzle body, after each injection this sleeve finds a slightly different position, and the shape of the guide gap (crescent gap-annular gap) varies from one injection even to another. Consequently, the quantity flowing into the coupler afterward during the reciprocating motion varies from one injection to another. These differences can become especially great whenever the following fuel quantity per injection is high, which is the case particularly with a long stroke of the injection valve member embodied in needle-like form and with high system pressure. Since the trailing fuel volume affects the closing motion and the closing instant of the injection valve member, this situation results in relatively major variations in the injection quantity from stroke to stroke.